By Ritsuko Ando and Daniel Leussink

A member of the Ugandan Olympic team who tested positive for the coronavirus upon arrival in Japan had the Delta variant, Japan's Olympic minister said on Friday, adding to concern the Games - less than a month away - may trigger a new wave of infections.

A coach in the African nation's delegation tested positive after arriving in Japan on Saturday, while a second member, an athlete, tested positive on Wednesday after arriving in the team's host city of Izumisano, officials said previously.

Olympic Minister Tamayo Marukawa told a news conference that the person who arrived on Saturday had been found to have the Delta variant, NHK public TV reported, adding that an analysis was also being conducted on the second confirmed case.

Marukawa said she would consult other ministries and liaise with those on the ground about what steps were needed, NHK said.

Japan has not suffered the explosive outbreak of the virus seen elsewhere but has struggled with a fourth wave of infections.

A decline in the pace of new cases and a speeded up vaccination rollout prompted it to ease a state of emergency in Tokyo and eight other prefectures on Sunday.

But experts have been expressing concerns about a renewed rise in cases in Tokyo as well as about the spread of more highly transmissible variants. Tokyo recorded 570 new COVID-19 cases on Thursday, up from 452 the same day a week earlier.

Japan's government and organisers have vowed to make the Games, which begin on July 23, "safe and secure". But many Japanese remain sceptical about the possibility of holding even a scaled-down Games safely during the pandemic.

Organisers have excluded foreign spectators and limited the number of domestic ones for the event. Alcohol, high-fives and talking loudly will also be banned at stadiums.

Economy Minister Yasutoshi Nishimura, the government's pointman on the pandemic response, said authorities needed to keep the Delta variant in mind, given the experience in the United States and Britain, where it spread rapidly. He said it currently accounted for 3% of new cases in Japan.

"Considering that the Delta variant will spread ... it is important to continue strong measures," he told reporters.

Nishimura said stronger steps would be taken if infections spread to a "certain degree" or hospitals were strained but a fresh state of emergency would not be immediately imposed.

Some areas including Tokyo remain under "quasi-emergency" restrictions, including limits on the sale of alcohol at eateries. A ban might need to be reimposed, Nishimura said.